Some electronic circuits impose an upper limit on a current that may be delivered to a load with the purpose of protecting the circuit or the load. For example, a universal serial bus (USB) has an upper current limit of 100 mA when it operates in a low power mode, and has an upper current limit of 500 mA when it operates in a high power mode.
FIG. 1 illustrates a conventional current limit circuit 100. The current limit circuit 100 includes a controller 116, a converter 120, and a switch 110, such as a metal oxide semiconductor field effect transistor (MOSFET). An input power source VIN is coupled to a load 102 through an output node VSYS via the switch 110. The controller 116 is coupled to the switch 110 and the output node VSYS to monitor a voltage at the output node VSYS. The input power source VIN is also coupled to a battery 104 at the node VSYS to charge the battery 104. A current flowing through the switch 110 can increase as a power demand of the load 102 increases. By monitoring the voltage at the output node VSYS, once the current flowing through the switch 110 reaches a predetermined upper current limit, the switch 110 operates in a linear mode to limit the current flowing through the switch 110. If the power demand of the load 102 keeps increasing, the input power source VIN alone may not provide sufficient power to the load 102. Consequently, the converter 120 can provide extra power from the battery 104 to the load 102.
However, by operating the switch 110 in the linear mode, it causes a thermal problem and extra power consumption.